The Freedom Valley Chronicles: Sinkholes Part Six – Surface Depressions

Surface depressions have been identified in Plymouth Meeting, neighborhoods along Colwell Lane, and throughout the Freedom Valley. In the map above, the orange dots represent surface depressions. The green dots are locations of sinkholes.

As noted in a previous edition of The Freedom Valley Chronicles, neighborhoods with known sinkholes are Plymouth Meeting Park, Whitemarsh Meadows, Joshua Knoll, Wainwright At Whitemarsh, Whitemarsh Green, Sandwood Village, Woodside Estates, and Farmview Village. Surface depressions have been officially identified in numerous locations, including on properties owned by the Colonial School District and Plymouth and Whitemarsh Townships. Among those properties are the campus of Plymouth Whitemarsh High School, Colwell Park, and Miles Park.

You can view additional maps that detail the locations of known sinkholes and surface depressions in specific neighborhoods in Plymouth Township and Whitemarsh Township by clicking here.

“Basically, a surface depression is a dip or sag on the land surface,” explained Mr. William Kochanov, Senior Geologist at the Bureau of Topographic and Geological Survey of the Pennsylvania Department of Conservation and Natural Resources. “However, not all surface depressions are related to karst features. They can be related to the underlying geology or to human activity.”

Karst features include limestone. This type of rock formation is found throughout the Freedom Valley.

Surface depressions can be precursors to sinkholes, and surface depressions can be unrelated to sinkholes, according to Mr. Kochanov.

Both are possibilities.

Mr. Kochanov provided a detailed explanation:

“Surface depressions may represent irregularities in the underlying bedrock caused by their orientation (e.g., whether the rock layers are folded, horizontal, or inclined) or to differences in the rocks composition (e.g., hard rocks vs. softer rocks; softer rocks would weather or erode more readily over time). Additionally, we know that carbonate rocks are chemically weathered by the dissolving action of naturally formed acids. Preferential weathering along the many cracks in carbonate bedrock would allow them to become more weathered than those areas where the cracked rocks are not as prevalent. These factors can help to produce highs and lows in the landscape.”

“In the context of the karst landscape, the depression also adds the characteristic of having internal water drainage – water generally flows into the feature but does not flow out. Think of them as drains. Meteoric water infiltrates into the ground and becomes groundwater. In karst areas, surface depressions and sinkholes help to facilitate the movement of groundwater to the water table.”

This aerial photograph shows an area just east and a bit north of Campbelltown, Lebanon County in 1957.This aerial photo shows the same area just east and a bit north of Campbelltown, Lebanon County in 1964. Note the darker gray spots on this image as compared to the map from 1957.

The two aerial photographs above both show the same areas of central Pennsylvania at different times.

“One of the images was photographed in 1957, the same area in 1964,” explained Mr. Kochanov. “The 1964 image clearly shows darker gray ‘spots’ on the land surface. These represent the karst drains. They retain the moisture of the soil longer because as a karstic surface depression, they hold water, [and] then allow the water to drain internally into the subsurface slowly over a period of time. If the photo was taken at the right time of year and under wet weather conditions (as in this case) these surface depressions can be easily observed.”

“Does this mean that surface depressions are precursors to sinkholes? Possibly,” Mr. Kochanov continued. “Just as with any plumbing system, drains and pipes can connect with one another to a common discharge point. Some pipes can be open but more likely they are clogged with soil. Remember, to get a sinkhole there has to be another opening or void in the subsurface for the surface material to move into in order to make a hole. On the other hand, the surface depressions can simply be dips in the bedrock and have no subsurface interconnections. We can see these features on the land surface, providing hints or clues as to what may lie beneath but we may not truly know the subsurface story until the landscape ‘rug’ is pulled back. Recognition of surface features such as depressions in areas underlain by carbonate bedrock [like limestone] makes us aware of potential subsidence or flooding (when the drains back up).”

“Human activity covers a lot of ground. Buried materials that … rot away, decompose, or deteriorate can create openings or voids in the subsurface and cause the land surface to sag. They may have nothing to do with karst and sinkhole formation but may provide important historical aspects about a property.”

The surface depressions identified on the map by the Commonwealth may eventually become sinkholes or they may be completely unrelated to sinkholes.

The top map is courtesy of the Pennsylvania Department of Conversation and Natural Resources, 2018.

The two aerial photographs of Lebanon County are courtesy of the United States Department of Agriculture, 1957 and 1964.